The present invention relates to frequency conversion devices, particularly those devices for converting television signals from a first frequency or channel to a second frequency or channel.
Community antenna television systems (CATV), master antenna television systems (MATV) and cable TV systems have been in existence for several years. One of the features common to each of these systems is the construction of a central signal distribution station or "head-end" site. From this location, all of the signals to be viewed by system subscribers are selected, processed to remove unwanted interferences, and distributed. The distribution of these channels usually occurs at frequencies different from the frequencies at which such signals were selected or received. For example, television signals at frequencies associated with channels 3, 6 and 10 could be distributed to subscribers at frequencies associated with channels 20, 22 and 24.
Previously, these types of systems distributed selected television programming on the channels on which they were received by the master antenna. For example, Wallerstein U.S. Pat. No. 3,366,731 discloses an MATV system wherey channels 2, 4 and 7 are received and processed by dedicated signal channel amplifiers. Thereafter, the individual channel signals, still at the same frequency, were mixed and distributed to sets of receivers. Particular channels would thereafter be selected by the tuners associated with each receiver. A similar type of system is shown in Oestreicher et al U.S. Pat. No. 2,570,475.
In present CATV, MATV and Cable TV systems, the problem of multiple dedicated components has intensified due to the distribution of programming on channels different from the channels on which respective signals were received. Accordingly, system owners were obligated to keep two components for each channel in inventory for use as a spare part, one for reception and one for transmission. To alleviate this problem, a number of devices were developed which were capable of selecting a single channel from a plurality of channel signals and additional devices which were capable of transmitting a signal on any one channel selected from a plurality of channels.
Walding, U.S. Pat. No. 3,882,266, shows such a device, which is capable of selecting a single channel from a range of channels at the input and providing a fixed channel output. This feature is accomplished in Walding by the provision of a variable tuner, including a filter-mixer-variable oscillator combination, at the input for selecting the desired channel and a fixed oscillator-mixer combination at the output. Walding also suggests reversing the combination, whereby the oscillator at the input is fixed and the oscillator at the output is made variable.
Yamashita et al, U.S. Pat. No. 4,315,333, discloses a VHF-UHF television superheterodyne receiver including first and second frequency conversion stages and an Intermediate frequency (I.F.) amplifying stage. Each of the first and second frequency conversion stages include a mixer, a local oscillator and an unbalanced/balanced conversion circuit interposed therebetween. The first local oscillator is said to be variable, i.e. variable tuner, while the second stage oscillator is fixed.
Ohta et al, U.S. Pat. No. 4,322,856, discloses yet another device for receiving television aural signals, which incorporates a two-stage device having a variable tuner, i.e. mixer-variable oscillator combination at the input for selecting either a VHF or UHF television aural signal and a mixer-fixed oscillator combination outputting the selected signal at a fixed frequency or channel.
Ma, U.S. Pat. No. 4,352,209, shows still a further frequency converter circuit including a variable tuner, a filter-mixer-variable oscillator combination, at the input which selects a desired television channel and up converts the signal to an Intermediate Frequency which is thereafter down converted by a mixer-fixed oscillator combination to a fixed output channel.
None of the references cited above include a variable tuned input for selecting a desired channel from a range of channels and a variable tuned output for transmitting the desired channel on a further selected channel.
Conventional design of frequency conversion devices for television signals is in a direction away from combining a variable tuner input with a variable tuner output because of the generation of interfering beat frequencies. To that end, Ohta, processing aural signals, discloses that the frequencies of the first and second oscillators need to be designed so as not to interfere with each other. Ohta, therefore, fixes the output oscillators so that an output frequency of 10.7 MHz is provided and the beat frequency between both oscillation frequencies is prevented from becomming 10.7 MHz. This teaching is believed to be confirmed by Ma who indicates that an overriding aspect of such system designs is to provide for isolation between first and second local oscillator signal frequencies to avoid generation of undesired interfering signals and signal products. Further, the April, 1972 Edition of Broadcast Engineering C 1972 by Howard W. Sams & Co., Inc., specifically states that certain channel combinations cannot be used because of interfering spurious signals, for example, channel 14 and 9, channel 14 and 5, channel 22 and 7 or channel 65 and 10 cannot be combined.
Accordingly, CATV, MATV or Cable TV systems still maintain an inventory of variable input dedicated devices and variable output dedicated devices.